An MS/MS analysis (tandem analysis), which is a technique of mass spectrometry, is a useful technique for identifying a high-molecular compound and/or analyzing its chemical structure. In recent years, this technique has been widely used in various areas. A structural analysis of a target compound using an MS/MS analysis is normally performed as follows: An ion having a specific mass-to-charge ratio originating from the target compound is selected as a precursor ion from various kinds of ions generated from a sample. The precursor ion is fragmented by an appropriate technique, such as making the precursor ion collide with CID gas. The thereby generated product ions are separated according to their mass-to-charge ratios and detected. Based on the detection signals, a mass spectrum (product-ion spectrum) which shows the relationship between the mass-to-charge ratios of the product ions and their intensities is obtained. The chemical structure of the target compound is estimated from the peak pattern of the product ions observed in the mass spectrum. In some cases, a neutral loss determined from the difference in mass-to-charge ratio between two peaks may also be used for the estimation.
A commonly known type of mass spectrometer for MS/MS analysis is a triple quadrupole mass spectrometer including two quadrupole mass filters respectively placed before and after a collision cell in which the CID process is performed. A so-called Q-TOF mass spectrometer, which can be obtained by replacing the rear quadrupole mass filter in the triple quadrupole mass spectrometer with a time-of-flight mass spectrometer, is more complex in structure and more expensive than the triple quadrupole mass spectrometer, yet has the advantage of being capable of acquiring more accurate mass spectra. In the present description, a mass spectrometer having two mass analyzers respectively placed before and after a collision cell is called the “tandem mass spectrometer”.
As is commonly known, various bonding sites in a compound have different levels of bond energy, so that those bonding sites also vary in ease of breakage. Therefore, if the collision energy which is determined, for example, by the voltage difference between a DC bias voltage applied to the front quadrupole mass filter and a DC bias voltage applied to an ion guide placed within the collision cell is changed in a tandem mass spectrometer, the fragmentation occurs in different forms even if the precursor ion originates from the same compound. In general, for an analysis of the chemical structure of a complex compound, it is convenient to have information on the masses of a wider variety of fragments (product ions and neutral losses) originating from the compound. Accordingly, in a conventionally known technique, a mass spectrometric analysis of the product ions is repeated for the same compound while changing the amount of collision energy to a plurality of levels. The obtained mass spectra are accumulated or averaged to create a mass spectrum in which a larger number of kinds of product ions can be observed, and a qualitative determination or structural analysis is performed using this mass spectrum.
A mass spectrometer described in Patent Literature 1 has been known as a device for obtaining, with a small number of analyses, a mass spectrum in which various kinds of product ions produced by fragmentation under different levels of collision energy can be observed. In this mass spectrometer, an ion trap for temporarily capturing ions is placed between a collision cell and a rear mass separator (time-of-flight mass analyzer). Product ions generated by fragmentation at different points in time under different levels of collision energy are temporarily stored in the ion trap. The stored ions are simultaneously ejected from the ion trap and subjected to mass spectrometry by the time-of-flight mass analyzer to obtain a mass spectrum. By this technique, a mass spectrum which shows various product ions that have resulted from fragmentation under different levels of collision energy, i.e. which have been obtained in various modes of fragmentation, can be obtained by a single mass spectrometric operation in the time-of-flight mass analyzer.